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I always thought it was more common in British than American English. But disregarding the implications of the word "legal" as applied to our language, I'm surprised OP would ask such a question of a usage which he admits "bothers him", and which he himself calls "a mistake". I'm even more surprised that the question title actually asks how such usage could have originated.
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FumbleFingersNov 18 '11 at 0:04

I too thought that this was an American failing until I moved to Europe and saw an English grammar review program on the BBC learning zone. It devoted several minutes to explaining why "could of" and similar are incorrect.
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phoogNov 18 '11 at 0:18

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@FumbleFingers people ask questions when they are curious to get answers. What's surprising about it? And I asked if this was 'legal English' because, although I think it's a mistake, I could be wrong and it could, infact, be grammatically correct.
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psychotikNov 18 '11 at 1:45

I think people write could of instead could've because it sounds like could of - it is a contraction of could have that is spelled could've but sounds the same as could of. The way it would be written without the contraction is I could have been hurt.

The phrase could of seems completely incorrect to me, as I can't even understand what it would mean.

That sounds logical - curious if there are other theories, and if this is unique to American English. If it is simply a phonetic contraction, I would assume other English speakers might do the same...
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psychotikNov 17 '11 at 18:45

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@psychotik: Strictly anecdotally, it seems to me that this can happen with English speakers anywhere.
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FrustratedWithFormsDesignerNov 17 '11 at 18:47

Sounds very plausible to me. I think we all make the mistake of using homonyms in our writing now and then. For example, writing "to" when you meant to write "too" or "night" when you meant "knight".
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BjornNov 17 '11 at 18:52

The last answer, is right. "people write could of instead could've because it sounds like could of - it is a contraction of could have that is spelled could've but sounds the same as could of."

This is normally done by people who have a limited education in the basics of English. In the UK, I have only heard children and foreigners use it. Mainly because, they learn their language skills mainly by copying the speech of other people. Therefore, they assume that it is written in the same way. This means they are ignorant of the important nuances in English